Money which is earning interest, investments or share income is often money which has already been taxed when earned.
That's specious argument. Money which pays VAT on purchases ,licence fees & renewal fees (driving licence, passport renewal for e.g) is also money which has been taxed when earned. There's little special treatment which exempts anyone from paying it.
The government creates and issues all our money, it taxes back a proportion of it to prevent inflation. There is no reason why it should make any sort of 'special case' for some of that money to be more lightly taxed on the grounds that it's already been taxed.
The 'deficit' between the money it has issued and the amount it taxes back is the amount of money in the economy which it hasn't yet taxed back. The deficit is essential for keeping money in circulation to drive economic activity and to accommodate a growing population. It also represents people's savings.
N.B. Money is created by banks, under licence from the government; as loans which are paid back, plus interest. Once the loan is repaid the money the bank created is essentially 'dead', it no longer exists, the interest is the bank's profit. Only money spent directly into the economy by the government leaves enough money in the economy, after taxation, to enable continuing economic activity.